The 74-year-old US senator from Vermont vows to work hard to make the former Secretary of State become president.

US Senator Bernie Sanders has withdrawn his presidential campaign and endorsed his one-time rival Hillary Clinton in the race to the White House, ending his long-shot bid as the Democrat nominee.

"Secretary Clinton has won the Democratic nominating process," Sanders declared on Tuesday in a joint appearance in the US state of New Hampshire.

After an often bitter campaign, Sanders said he will do everything he can to make Clinton "the next president" of the US.

"This campaign is about the needs of the American people," Sanders added, echoing some themes of his own year-long campaign.

He also criticised Clinton's presumptive Republican opponent Donald Trump for his "reckless economic policies", which he warned could lead to trillions of dollars of debt.

The 74-year-old Sanders, a US senator from Vermont, waged a tougher-than-expected year-long battle against the former secretary of state, but Clinton clinched enough delegates to secure the nomination in early June.

Sanders, a feisty self-described democratic socialist, nevertheless has long resisted conceding defeat to his rival, although he has said he would vote for Clinton and do anything to help defeat Trump.

On Tuesday, Trump said Sanders "totally sold out" to Clinton. Trump is expected to announce his vice presidential pick in the next few days.

Sanders wants to ensure that his ideas are part of the party platform presented at the Democratic National Convention later this month in Philadelphia, when Clinton is formally nominated.

Party officials met over the weekend in Orlando, Florida to finalise the Democratic platform, which they described as the most ambitious and progressive yet in history.